In this paper a nonlinear suboptimal guidance and autopilot design is presented for the powered braking phase of the autonomous soft landing of lunar module. The design objective is to drive the spacecraft from perilune (a point on the lunar orbit closest to the lunar surface) to a designated location over the lunar surface while ensuring the terminal touchdown velocity and the terminal vertical orientation of the spacecraft. In general the autonomous guidance generate the acceleration command based on the point mass dynamics. In order to follow the desired trajectory generated using guidance module, a three loop structure based closed loop guidance and control design for the spacecraft rigid body dynamics is presented. The nonlinear dynamic inversion based tracking controller is introduced to follow the desired body attitude. The intermediate loop is designed generate the desired body rate commands which are again tracked by the inner most loop to provide the necessary torque along the body axis. To realize the torque requirement small attitude thruster based reaction control configuration is presented. The reaction thrusters are in general on-off thrusters with discrete actuation capability. Using the appropriate thruster selection logic the reaction thruster are activated in proper sequence to achieve the required torque demand. Based on the proposed design a simulation study has been carried out with six degree of freedom spacecraft model and effectiveness is demonstrated with simulation results.
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